HP 7475A Plotter: OEM Pen Body Model

You can refill the HP pens with liquid ink and continue plotting until the fiber tip wears out. That would limit me to the CMYK inkjet inks on the shelf, although I suppose investing in drafting inks might be amusing.

However, it should be feasible to build an adapter to hold an ordinary, albeit skinny, drawing / drafting pen, perhaps chopped down to be only a bit longer than the OEM plotter pens. That has the advantage of using cheap & readily available materials, doesn’t require much capital outlay, and, come to think of it, gives me a Digital Machinist column topic… [grin]

There’s a vague notion of converting the plotter into a vinyl / paper / stencil cutter, although I expect the snap-in pen holder can’t exert enough lateral force to hold a cutting knife in position, nor enough downward force to push the blade through the vinyl / paper / whatever. But ya never know until you try.

So, we begin…

A bit of digital caliper work provides a list of points defining the OEM pen body outline:

Rather than computing the radius by hand, it’s easier to just divide the easily measured diameter by two and be done with it.

The point array defines a polygon in the XY plane:

HP7475A – HP Plotter Pen Body – plane polygon

Then you feed that polygon into a rotate_extrude(), which spins up a reasonable simulacrum of a plotter pen:

HP7475A – HP Plotter Pen Body – solid model

I picked the coordinates to put the tip at (0,0,0) and converted the tapered fiber nib into a plain cylinder.

That shape is obviously impossible to print without vast amounts of support, but splitting it across the middle of the flange and rearranging the pieces works just fine:

HP7475A – HP Plotter Pen Body – build layout

A pair of alignment pin holes simplifies gluing the parts back together:

HP7475A – HP Plotter Pen Body – solid model – bottom

There’s a subtle problem lurking in that flange, which is 2.0 mm thick at the base and 1.0 mm thick at the rim. Splitting it in half requires each part to build correctly from an integral number of thread layers, so you must use a thread thickness (that’s in the Z direction) that divides evenly into the required height. I’ve been using 0.2 mm, which would produce a 1.2 mm rim.

Slicing at 0.25 mm produced a 2.1 mm flange with a 1.1 mm rim, suggesting that:

I could apply a Slic3r Modifier Mesh to print the flange with 0.10 mm layers, but that seems like entirely too much effort right now.

At the other end of the model, converting the tapered tip into a blunt cylinder didn’t save it from melting down:

HP 7475A Plotter Pen – solid PETG

It might be possible to reduce the printing speed enough to produce that tiny cylinder, but I needed just the upper body to verify that it fit correctly into the carousel:

HP 7475A Plotter Pen Body – in carousel

As you’d expect, the rubber boots that used to seal the pen tips have long since rotted out:

HP 7475A Carousel Rubber Boots

You can find sources for those boots, but at $252 (marked down to $144!) each, perhaps it’d be more feasible to gimmick up a two-part mold and cast silicone rubber duplicates; I could sell a set of six for $200 and get rich. Heck, I could even undercut their $40.32 two-year protection plan by a considerable margin.

Anyhow, the pen holder plucked it out of the carousel just like a real HP pen:

HP 7475A Plotter Pen Body – in holder

Note that the carousel and pen holder contact the flange and the cylindrical body, not either of the tapered sections down to the tip. That means I can carve away the entire bottom part of the body to make a drawing pen adapter…

The OpenSCAD source code includes a bunch of features & parts I’ll describe in the next few posts, but which certainly should not be regarded as final copy:

One thing I considered before I gave mine away was replacing the pen with a powerful laser diode and lense, and using it as a laser cutter. It would only cut cloth, paper, and perhaps thin cardboard, but cutting with light avoids all the torque/drag/backlash issues with a knife. I figured I could grab the pen up/down signal and use it to turn the laser on and off.

It does: I ended up doing that briefly with my 7470. I used a microswitch to turn it on and off. If I had to do it again I think I’d splice into the pen up/down electrics.
The drag was that on anything light-colored, the laser wouldn’t start burning fast enough. Once there’s a black spot everything’s fine, it runs that spot right along, but you have to dwell for about half a second at the first point to get it started. That’s one reason I went to doing this all in the mill, where I can use a digital output to control the laser, and use pauses to give it time to get the spot started.

(both machines do a nice job of cutting gasket material for engines: it’s dark enough to start burning immediately, and when it gets done I can just snap them out. But the mill is so much more convenient.)

I took a different tack, and bought one of those cheap Chinese laser cutters with a 40 watt CO2 laser. First thing, I repaired the laser power supply. Then, I replaced the control electronics with an EiBot board, so I could actually talk to it (the electronics it came with contained already-oozing capacitors, and a Champ-36 connector apparently intended to connect to a line printer port using an undocumented protocol). Now can run the laser cutter directly from InkScape over an ordinary USB connection. I’m currently in the process of adding air assist to blow the smoke away as well as encourage melted acrylic to not stick around.

Looks like Amazon has them for $500-ish these days. The reviews suggest the cutter arrives as a shop project, but if you can get past that, it works reasonably well.

I’m not quite ready for one right now, but it seems more do-able (albeit much smaller) than a Lasersaur and much, much safer than the “clip a laser diode to your Reprap printer” products on Amazon… [sigh]

There’s an InkScape plugin (written in Python) that drives it directly. The laser cutter has a X and Y homing switches, so I’m thinking of modifying the plugin to jog to a known position before cutting. Fortunately, the EiBot board is based on the USB BitWhacker, and supports those functions as well, so I can monitor input leads for switch closures.

One a day, every day…

Redshifting

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